Check for duplicates in list

some_list = ['a', 'b', 'c', 'b', 'd', 'm', 'n', 'n']

unique = []
duplicates = []

for item in some_list:
  if item not in unique:
    unique.append(item)
  else:
    duplicates.append(item)  

print(unique)
print(duplicates)

######################

duplicates = []

for item in some_list:
  if some_list.count(item) > 1 and item not in duplicates:
    duplicates.append(item)  

print(duplicates)